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There was a discussion about this about a month ago on Reddit. Redditor Updatebjarni had a nice concise explaination:
"A modern CPU uses a circuit technology called CMOS. A property of CMOS circuits is that they have a low static power consumption, meaning that if the state of an individual gate doesn't change, the gate consumes very little electrical current, and thus power. A little current is needed each time the gate needs to flip to the other state, and so the more often you change the state of the gate, the more power it consumes.
When a CPU is idle, it spends relatively less of its time computing anything, meaning that in a given time the gates change state fewer times. The CPU can also lower its clock frequency to further decrease the frequency of state transistions, and will also actively use a smaller portion of the chip, which contains lots of parallel circuits to enable better performance under stress by parallelising the work over multiple work units."